1. Field of the Invention
The present invention relates to composite structures manufacturing methods and apparatus for practicing such methods and, more particularly, to manufacturing composite structures using preimpregnated fibrous materials in fiber placement and/or tape laying systems.
2. State of the Art
Prior to the present invention, fiber placement systems generally consisted of placing rovings or tows impregnated with either thermoplastic or thermosetting-type resin as a single tow or an entire band of tows in a number of superimposed layers onto a workpiece on a mandrel to produce a geodesic-shaped article. This geodesic-shaped article was then placed in a bag, a vacuum was then created in the bag, and the bag was placed in an autoclave and heated at high temperatures and pressures in order to cure and consolidate the article.
Another technique used in the art to cure and consolidate composite structures uses only an oven rather than an autoclave. Autoclave curing requires constant monitoring and additional safety precautions as well as being more expensive in operation, while oven curing does not require constant monitoring or additional safety precautions and is cheaply operated. This technique places fiber tows using a wet filament winding technique where the composite material would gel at room temperature and then the component part is cured in an oven.
This previous technique produced parts that had high void levels (i.e., up to 10%) when oven cured, or required additional handling to produce low void levels parts (i.e., less than 2%). In the wet winding technique, an excess of resin had to be used, followed by a squeegee technique, or vacuum consolidation and gas removal during the oven cure to produce a low void part.
Both of the prior art fiber placement and wet winding processes have disadvantages. Wet-winding, while offering the advantage of simple oven curing, is not conducive to fiber placement. The versatility of fiber placement is its capability of winding complex shapes using more of a variety of winding angles, changing angles while winding, and cutting and adding tows on demand. Fiber placement, on the other hand, needs high-performance prepreg systems, and requires consolidation using a vacuum bag and autoclave pressure during curing. Also, in the fabrication of very thick structures where many layers of fiber are superimposed on each other, waves in the superimposed layers are normally formed because of debulking (i.e., elimination of voids) during curing of the structure in an autoclave.
The present invention overcomes the disadvantages of the prior art by substantially curing the fiber as it is being laid down on the workpiece on the mandrel in a system to be marketed under the trademark xe2x80x9cCure-on-the-Flyxe2x80x9d.
The present invention is directed to a method of producing substantially cured fiber laminations in situ while laying up fiber on a mandrel comprising:
a) impregnating at least one fiber tow or tape with at least one thermoset resin,
b) passing the at least one impregnated tow or tape to a preheating zone for preheating the tow or tape to a predetermined temperature based on the particular thermoset resin in the range of from ambient to about 375xc2x0 C. where the curing of the tow or tape is partially advanced, and
c) laying up the partially advanced, preheated tow or tape onto the mandrel while simultaneously advancing the curing of said tow to substantial completion (i.e., greater than 60%) by
i) supplying additional heat simultaneously to the area of the mandrel proximate to where the fiber is being placed thereon,
ii) monitoring a plurality of parameters characteristic of said fiber placement apparatus, and
iii) controlling the amount of advancement of cure of the resin in the fiber as a function of the monitored values of said predetermined parameters.
This invention also comprehends a fiber placement system for performing the above-mentioned method comprising:
a) means for impregnating at least one fiber tow or tape with a thermoset resin,
b) means for preheating the at least one impregnated tow or tape to a predetermined temperature within an advancement to cure range for a predetermined time period based on characteristics of the resin for partially advancing the resin to cure,
c) means for laying up the preheated, partially advanced to cure tow or tape onto a mandrel, and
d) nip-point heating means associated with at least one parameter monitoring means located proximate to the laying up means for simultaneously heating the tow or tape to a temperature within the curing range of the resin while the tows are being placed on the mandrel and the curing of said tow or tape are being further advanced to substantially completion.